Mixers may be used in a broad range of applications involving the mixing of fluids with other substances. For example, in the chemical processing industry, mixers may be utilized to homogenize and emulsify various multi-phase fluids. In one particular application, mixers are utilized to mix clay kaolin into water for use in making a number of products including paper and ceramics. In another application, mixers may be used to mix oxygen with waste products to facilitate biodegradation. In these processing systems, mixing is executed on a batch-by-batch basis in large retention tanks or towers. Since these methods require that a batch of substances be isolated for a given period of time while being mixed, either large numbers of retention tanks are required or the overall output of the processing plant is limited.
Mixers have also been used in the paper-making industry to mix fibrous pulp slurries with bleaching chemicals such as sodium or calcium hypochlorite, chlorine gas, hydrogen dioxide, etc. Some paper processing plants utilize continuous feed mixers to mix chemicals into fibrous slurries on a continuous basis as the slurries move through the processing system. These previous mixers work with slurries or suspensions having varying consistencies of wood pulp content, ranging from less than 1% to nearly 20% by weight. Some of the continuous feed mixers do not include moving parts but force the fibrous slurry and the mixing chemicals into narrow passageways which include sharp corners causing the direction of flow to change abruptly thereby disrupting the fibrous suspension and chemicals and causing them to mix. These types of mixers have been utilized in most cases to mix chemicals with fibrous slurries having consistencies in the lower range from below 1% to 4% by weight.
At certain points in the paper-making process, or at all points in certain processes, it can be advantageous to maintain the suspension at a high consistency. In such situations, it becomes more difficult to properly mix the chemicals into the fibrous slurry on a continuous basis. This is due to the tendency of the fiber suspension to create flocs or clumps of fibers at higher consistencies which inherently entangle to form networks or structures. To achieve a uniform distribution of chemicals in these situations without lowering the consistency of the fibrous suspension, the fibrous network must be disrupted.
Previous continuous mixers for mixing chemicals with higher consistency fibrous suspensions impart turbulence into the fibrous slurry and chemicals to effect mixing. Examples of these earlier mixers are shown in U.S. Pat. Nos. 3,887,429 and 3,559,957 and Canadian patent 1,102,604. In each of these earlier mixers, a rotor including protrusions along its outer circumference rotates within a chamber having protrusions which extended in an opposed direction. As the rotor rotates, shear forces are generated between the rotor protrusions and the chamber protrusions thereby disrupting the network of the fibrous suspension. In some of these earlier devices, the axis of rotation of the rotor is parallel to the overall flow of fibrous suspension and chemicals through the mixer. In other mixers, the axis of the rotor is perpendicular to the overall flow of the substances through the device. In either case, these earlier mixers focus on imparting turbulence to the slurry in order to disrupt the network and allow the chemicals to become mixed with the fibrous suspension.
While the earlier mixers can successfully mix chemicals into a fibrous slurry on a continuous basis, they have certain drawbacks and deficiencies. One important overall shortcoming of the prior mixers is that they require a substantial amount of energy to maintain an adequate rate of flow for a given degree of mixing. In addition, the earlier mixers create a substantial pressure drop across the mixer as measured at the inlet and outlet. These inefficiencies among others explained below lead to higher costs of manufacture and operation and added complexity of machinery, installation and service.
A specific shortcoming of the prior mixers is that the rotating device, in generating shear forces alone, requires a significant amount of power not contributing to the flow of substances through the mixer or reducing the pressure drop from the inlet to the outlet of the mixer. As a result, the pressure at the inlet of the mixer must be maintained at a higher level for a given outlet pressure. Consequently, a larger, more costly, pump must be utilized upstream of the mixer in order to maintain the higher inlet pressure.
Another related shortcoming of prior mixers arises from the fact that each typically has a chamber with an interior surface which is concentric with the axis of rotation of the rotor. This concentricity does not facilitate flow through the mixer while generating turbulence. More specifically, the prior mixers include protrusions symmetrically placed along the inner surface of the casing such that substantially equal amounts of turbulence are generated against the direction of the overall flow of substances through the mixer as in the direction of flow. Consequently, the flow components generated by the rotor element of the previous mixers essentially cancel each other which results in a higher overall resistance to flow through the mixer. Stated another way, the rotation of the mixer's rotor does nothing to facilitate flow through the mixer.
Another shortcoming of the prior mixers is that the rotating device includes substantial structure located directly in the overall flow path of the suspension through the mixing chamber. More specifically, the prior mixers include rotating elements having solid cylindrical bodies with protrusions on their outer surfaces and shafts extending through the chamber of the mixer. Rather than merely facilitating the mixing of the fibrous suspension with the chemicals, these prior mixers present considerable resistance to the flow of the substances through the mixer.
Yet another shortcoming of the prior mixers is that the protrusions which extend from the continuous outer surface of the rotating element only generate turbulence with the protrusions on the inner surface of the casing. The earlier mixers thus do not advantageously utilize any interaction between the protrusions of the rotating device. Consequently, the overall turbulence generated by the mixer is limited and more power and more structure is required.
Yet another shortcoming of the prior mixers is that the shear forces are generated between protrusions which are parallel to each other and with the axis of rotation of the rotor. Consequently, the entire length of the protrusions of the rotor and the chamber interact with one another at the same time. Thus, the shear forces generated by the interaction of the protrusions are relatively concentrated in time rather than being continuous and varying in nature.